Micro-display projection systems using a reflective light engine or imager, such as, for example, a digital light processor (DLP™) (Texas Instruments) imager, are increasingly utilized in color image or video projection devices (e.g., rear projection television (RPTV), home theater front projection TV and theater systems). In an existing projection system, shown in FIG. 1, a light source 10 is provided, in this case a UHP lamp, and generates white light (i.e., all color spectrums). Light from the light source 10 passes through a color wheel 20 which has a plurality of dichroic filtering elements, each of which allows a light band of one of the colors: blue, green or red to pass through and reflects light of the other colors. The color wheel 20 is rotated so that a temporal pattern of blue, green, and red light bands pass through the color wheel. The color wheel is typically rotated fast enough to create at least one primary color period for each primary color during each frame of a video image.
An integrator 30 receives the light band from the light source 10 that is allowed to pass through the color wheel 20 and directs the light band through relay optics 40 into a total internal reflection (TIR) prism 50. The TIR prism 50 deflects the light band onto an imager 60, such as a DLP imager. The imager modulates the intensity of individual pixels of the light beam and reflects them back through the TIR prism 50 and into a projection lens system 70. In DLP only the flash rate and flash width is modulated giving the impression of intensity modulation. In LCD and LCOS the intensity is modulated directly. The projection lens system 70 focuses the light pixels onto a screen (not shown) to form a viewable image. A color video image is formed by rapid successive matrices of pixels of each of the three colors (blue, green, and red) which are blended by the viewer's eye to forth a full color image.
Throughout this specification, and consistent with the practice of the relevant art, the term pixel is used to designate a small area or dot of an image, the corresponding portion of a light transmission, and the portion of an imager producing that light transmission.
The DLP imager 60 comprises a matrix of micro-mirrors, moveable between an angle that reflects light through the TIR prism 50 and into the projection lens system 70 at an angle that deflects the light so that it is not projected by the projection lens system 70. Each micro-mirror reflects a pixel of light of a desired intensity depending upon a succession of flashes of that particular micro-mirror which in turn are responsive to a video signal addressed to the DLP imager 60. Thus, in the DLP imager 60, each micro-mirror or pixel of the imager modulates the light incident on it according to a gray-scale factor input to the imager or light engine to form a matrix of discrete modulated light signals or pixels.
Until recently, one micro-mirror has been used for each pixel of the projected image and the grid of the array of square mirrors has been aligned parallel to the image edges. However, using Smooth Picture™ (Texas Instruments) technology, the grid of the micro-mirror array is rotated 45 degrees with respect to the image edges and the pixels appear as diamond shapes. The image formed by such an array is then displaced by a moveable optical component (mirror or lens) so as to appear in two sequential positions displaced by one half of the square pixel diagonal. Half of the picture detail is displayed in each of the sequential images. This allows resolution to be double the micro-mirror count. Micro-mirror count can be reduced allowing a lower cost imager for low end projectors while maintaining resolution. For high-end products (digital cinema or high end television products) the mirror count can remain high and the resolution can be doubled.
The use of the color wheel introduces a visual artifact called the “Rainbow Effect”. Since the color flashes sequentially, the color of the still object on the display will separate on the retina of a person's eye during rapid eye movement (eye dart) which can be up to a rate of 800 degrees per second. The brain perceives this separation as multiple primary colored images of the object with differing luminance. The luminance bandwidth of the visual system is wide but it also has a low pass characteristic. As the speed of color flashing is increased, the angular separation of the retina images decreases and the sensitivity to luminance variation decreases. Above about 3000 color flashes per second, assimilation occurs and the rainbow images appear to merge. Present single imager DLP™ systems run at less than 1000 color flashes per second so the rainbow effect is seen.
Color flash speed is difficult to increase because micro-mirror physical mass limits the maximum off-on-off flip time which sets the least significant unit of digital grey scale without the use of dithering. With the 8-bit grey scale range used in home television projectors the maximum flash time is 255 times larger. For Smooth Picture™ each color must light each micro-mirror at least twice per 16.6 mS image. The flashes are distributed by increasing the number of color wheel segments to increase the flash rate to 6 or 9 flashes per 8.33 mS or 720 to 1075 flashes per second.
Existing DLP imagers, however, suffer from several problems. The color wheel wastes light, as the light having the colors that are reflected by the color wheel is typically lost. Also, color separation or break-up artifacts degrade the image quality of the projection system, as described above. As such, a system for reducing color separation or breakup artifacts and/or having improved resolution is needed.